AUDIOCLINIC (Aug. 1987)

Faint Noise

Q. I recently discovered that my sound system produces a small amount of hum even when there is no signal source connected. The hum can be heard only when the ear is brought close to the loudspeaker and only when the surroundings are quiet. Is this normal?

-W. T. Lim, Republic of Singapore

A. Any piece of electronic equipment produces hum and noise, and you can often hear it if your speakers are efficient enough and you are close enough to them. It is only when this noise intrudes upon the music that you have a real concern.

Dealing with Dust

Like David Coup, whose problem was described in the November 1986 "Audioclinic," I have had problems with rotary controls becoming scratchy and jacks and plugs losing contact in a dry climate. I did not have this problem during the two years I lived in Florida despite the high concentration of salt in the South Florida air.

I have come to the conclusion that these scratchy control problems in dry climates are caused not by corrosion--as you speculated--but by dust.

Here in Phoenix, everything tends to get covered with fine dust. Probably because of electrostatic forces, adherence of this fine dust layer is very strong-enough so that it does not blow off my car when I drive it at 70 mph! Dust does not accumulate as much in Florida because of the high humidity and frequent rain, which tend to purify the air and keep the dust down.

(Incidentally, your premise that corrosion is "water combining with oxy gen" is close but not correct. Corrosion, as most laymen know it, is really a process where elemental oxygen com bines with a metal to form a metal oxide, which sometimes reduces electrical conductivity to zero. Water or high humidity will increase the rate of oxidation because there is more oxygen in moist air than in dry air.) To reduce dust buildup on electrical contacts (and to reduce or eliminate salt ingestion by components located near a beach), I recommend keeping the environs of any audio equipment very clean. At an extreme, this may mean never opening the windows; at a minimum, it will entail housing the equipment in a cabinet with few openings. (Remember, dust and salt can enter through vent openings.) Ventilation fans installed to cool components should have fine -particulate filters attached to catch dust before it enters the cabinet. This procedure has virtually eliminated my problem.

In closing, your recommendation of using WD40 to clean contacts is good.

The oily residue reduces the adherence of dust by forming a low shear-strength film on contact surfaces, allowing the contacts to plow through the dust as switches, relays, or plugs are operated. Periodic removal of the residue with a solvent (possibly Freon) is recommended because the oily WD40 will allow a buildup to occur. After Freon cleaning, recoat sparingly with WD40 or any light machine oil.

-Barry S. Draskovich, Phoenix, Ariz.

Hum Interference from TV Sets

Q. I have a humming noise caused by my power amplifier. The noise does not change even if all components other than the power amplifier are turned off. When my neighbor turns his television set off, however, the humming stops! Why does his TV affect my audio equipment? How can it be dealt with?

-Jonas Hillergren, Stockholm, Sweden

A. TV sets often radiate electromagnetic interference from their horizontal sync circuits, and sometimes this interference is introduced into the a.c. mains. I believe that this is producing the hum in your power amplifier.

Some equipment tends to be particularly sensitive to undesired signals on the power line. Narrowing the overall bandwidth of the amplifier can some times make these undesired signals inaudible. I realize that this is not a "purist" solution, although it is often the most practical one.

An alternate solution (which does not always produce good results) is to place a suitable LC filter between the wall plug and the power cord of the offending TV set. You may also need to place a similar filter between the a.c. outlet and the wall plug of your power amplifier. Be sure that the filters can handle the wattage drawn by the equipment connected to them.

Mismatched Sound in Speakers

Q. I recently purchased a pair of loudspeaker systems. I discovered that one speaker had a sloppier, more pronounced bass response and sounded louder than the other speaker. I notified the dealer, who immediately replaced the pair. Unfortunately, I am experiencing the same problem with these replacements.

I had my receiver checked, and it was found to be functioning properly. I have switched the cables around and even have used the "B" output terminals on my receiver; the problem persists. I don't want to face the dealer again! What do I do now?

-Joe Paolucci, Jr., Albany, N.Y.

A. This difference in sound between two supposedly identical loudspeakers may actually be not a speaker problem but one of room acoustics. The acoustical properties of a listening room can vary tremendously from one loudspeaker location to another. Physically switch the positions of your two loud speakers. If I'm right, you will find that there are areas of your listening room in which a loudspeaker--any loudspeaker--will always produce more bass and appear to be louder than the other speaker.

There is also a possible electrical explanation, not related to defective loudspeakers. If one speaker is placed very near to your sound system and the other one is located at a consider able distance, differences in the lengths of the interconnecting speaker cables could be responsible for the change. This is very likely in instances where one cable must be made excessively long. The problem can be aggravated if you are using a small-gauge wire, such as No. 22 (which is sometimes sold as "speaker wire").

The resistance in long, small-gauge wires can reduce the damping; if your loudspeaker system is subject to wild cone excursions when damping is too low, this could be the cause of your problem. You should replace this cable with something considerably heavier, such as No. 16 or No. 14.

If you have a problem or question about audio, write to Mr. Joseph Giovanelli at AUDIO Magazine, 1515 Broadway, New York, N.Y. 10036. All letters are answered. Please enclose a stamped, self-addressed envelope.

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(Source: Audio magazine, Aug. 1987, JOSEPH GIOVANELLI)

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